The present invention relates to a method of pre-treating secondary fibers prior to bleaching.
It is known when bleaching secondary fibers to allow the pulp to pass through a disperser in order to disperse any ink particles or like particles that may remain. After having passed through the dispersion stage, the pulp is transported to a bleaching stage, in which the bleaching chemicals peroxide, alkali and silicate are added. This known process consumes a large amount of the relatively expensive peroxide. It is also known to bleach secondary fibers by supplying oxygen to the system.
The known bleaching processes in which oxygen is used are relatively complicated. The oxygen treatment process is effected in a separate bleaching stage, in a pressurized reactor with a reactor residence time as long as at least ten minutes. The oxygen treatment process is also effected under alkaline conditions and at high pressure. One kilogram of peroxide is about ten times the price of oxygen. It is therefore desirable to keep the consumption of peroxide down to the lowest possible level and to replace peroxide with, for instance, oxygen to the greatest possible extent. Normally, it is not difficult to achieve desired pulp brightness when bleaching. When problems concerning brightness occur, it may be because the secondary paper contained a large percentage of mechanical pulp, large quantities of printing ink or not readily bleached ink or colors. Consequently, it is not only important to maintain a low peroxide consumption level, but also to increase brightness of the pulp in the bleaching process. EP-A-557 651 discloses a process for treatment of secondary fibre pulp. This known process is adapted to clean and treat secondary fibers of various qualities in one single process. This document discloses a screw digester, i.e. a digester which comprises a feeding screw and in which the pulp is subjected to chemical pulping. During the chemical pulping, usual chemical additives, i.e. NaOH, H.sub.2 O.sub.2 and O.sub.2, are added. The pulping takes place under high pressure, 70-135 psi (490-950 kPa) and at a temperature of 70-110.degree. C. The residence time in the pulper is at least 30 min. The purpose of the feeding screw of the screw digester disclosed is to transport the pulp from the inlet and to the outlet end and not to mechanically process or mix the pulp. Furthermore, it is not mentioned in EP-A-557 651 that the treatment in the screw digester would result in a uniform distribution of the ink particles or the added oxygen in the secondary fibre pulp.
U.S. Pat. No. 4,909,900 discloses the treatment of a fibre pulp mixture, which may include secondary fibers, by means of a disperser. Example 1 refers to reductive bleaching by sodium dithionite and example 2 to oxidative bleaching by hydrogen peroxide. It is not mentioned that oxygen may be used together with the bleaching technique disclosed in this document. In example 2 it is more closely disclosed that a solution comprising one percent of hydrogen peroxide is delivered to the pulp by means of a nozzle in a feeding conduit from which the pulp is introduced to a feeding screw transporting the pulp to the disperser. Thus, the hydrogen peroxide is added to the pulp at a position upstream the disperser in which the mechanical treatment of the pulp takes place.
U.S. Pat. No. 5,234,544 discloses the inking and bleaching of secondary fibre pulp by means of oxygen and particularly that the pulp is brought into contact with alkali, oxygen and a surfactant in a pressurized reaction zone. The residence time in the reaction zone is relatively long. In Example 1 it is disclosed that the pulp is supplied to the reactor at a relatively low consistency of 10% and that the pulp is retained in the reactor and periodically mixed therein by means of a high shear mixer.